92 NITRIC ANHYDRIDE, OR ANHYDROUS NITRIC ACID. seem to be due, however, to the existence of any true hydrate, but to the particular relation of the density of the vapour of nitric acid to that of water under a pressure of 30 inches of mercury; for by conducting the distillation at reduced pressures, Roscoe found that the density of the acid in the retort has no real fixed point, and that an acid may be thus obtained which does not correspond in composition to any definite hydrate, but that the proportion of water varies with the pressure under which the boiling takes place. The following table indicates the per-centage of nitric anhydride, N., often called dry nitric acid, contained in aqueous solutions of nitric acid of various specific gravities :: (362) Nitric Anhydride, or Anhydrous Nitric Acid (NO). Fusing pt. 85°; Boiling pt. 113°.-This substance is a very unstable compound, which may be obtained in the form of perfectly transparent, brilliant, colourless crystals, derived from the right rhombic prism; they melt at 85° and boil at 113°: at about the temperature last-named the compound begins to undergo decomposition. Sometimes the crystals, even if kept in sealed tubes, become decomposed at the ordinary atmospheric temperature, and the tube bursts with a dangerous explosion from the pressure exerted by the liberated gases. The crystals are dissolved rapidly by water, emitting much heat, and producing ordinary hydrated nitric acid. In order to procure the anhydride, a uniform current of per NITRIC ACID-COMMON IMPURITIES. 93 fectly dry chlorine gas is transmitted very slowly over crystals of well-dried nitrate of silver: the salt is at first heated to about 200° till the decomposition has commenced, and the temperature is then lowered to about 150°. The operation is one of considerable delicacy, and requires attention to a number of minute precautions, for the details of which the reader is referred to Deville's paper (Ann. de Chimie, III. xxviii. 241). The chlorine displaces the nitric acid radicle from the nitrate of silver; chloride of silver is formed, and the radicle breaks up into nitric anhydride whilst Oxygen escapes. By surrounding the receiver with a freezing mixture, the nitric anhydride is condensed in crystals. The decomposition may be represented in the following manner, though it is probably not quite so simple* :· Nitrate of silver. Chlor, silver. Nitric anhydride. 4 AgNO3 + 2 Cl, give 4 AgCl + →1⁄2 + 2 N205. 2 2 5° Deville ascertained the composition of nitric anhydride by estimating the quantity of nitrogen which a given weight of the compound furnished after the oxygen had been removed from it by transmitting its vapours over finely divided metallic copper, which, at a high temperature, combines rapidly with the oxygen. parts by weight of the anhydride were thus found to contain 25'9 of nitrogen; the deficiency, 74'1, is oxygen: or 28 parts of nitrogen are united with 80 of oxygen. 100 (363) Common Impurities of the Acid.-The nitric acid of commerce is liable to be contaminated with a variety of foreign matters, of which sulphuric acid, chlorine, potash, and oxide of iron are the most frequent. Its usual yellow or red colour is owing to the presence of some of the lower oxides of nitrogen. If pure, it leaves no fixed residue when evaporated on a slip of glass, and gives no precipitate when, after dilution with three or four times its bulk of water, it is tested for sulphuric acid with nitrate of barium, and for chlorine with nitrate of silver. By distilling it a second time, it may readily be obtained of specific gravity 142, and free from all impurities except the lower oxides of nitrogen. If chlorine be present, nitrate of silver may be added so long as the silver salt occasions a precipitate, or a silver coin may be dissolved in the acid; after which the rectification may be proceeded with. The lower oxides of nitrogen may be removed by diluting the acid with water till of a sp. gr. not ex = *Possibly a nitric oxychloride is formed first, thus:-2 AgN‡, + 2 Cl, 2 AgCl + 2 NO2Cl + 2, and then this nitric oxychloride decomposes a second atom of nitrate of silver; AgN‡, + Nе,C1 = AgCl + N2O5. ceeding 142, and then distilling with 2 or 3 per cent. of the acid chromate of potassium. Nitrates.-Nitric acid is monobasic; that is to say, each atom of acid requires one atom of a monad metal like potassium to neutralize it; the salts which it forms are termed nitrates. Their general formula is M'NO. These salts may be procured without difficulty by dissolving either the metal itself, or its oxide, or its carbonate, in nitric acid more or less diluted. Many of the nitrates, including those of potassium, sodium, ammonium, barium, lead, and silver, are anhydrous. Others combine with 6 atoms of water of crystallization; among these are the salts of magnesium (Mg 2 NO ̧ . 6 H2O), zinc, nickel, cobalt, iron, and copper; whilst in others the proportion of the water is different, nitrate of calcium retaining 4 H2O, and nitrate of strontium 5 H2O. If crystallized at a high temperature, nitrate of copper retains only 3 H2, and nitrate of strontium may be obtained in the anhydrous form. No acid nitrates are known to exist, but several basic nitrates may be procured; that is to say, salts may be formed which contain more than one equivalent of base for each equivalent of acid: such, for instance, as basic nitrate of copper, (Eu 2 NO ̧ . 3 €uᎾᎻ Ꮎ ) . 2 The following table gives the composition of some of the nitrates: Most of the nitrates fuse readily when heated: at an elevated temperature they are all decomposed. At first, from the nitrates of the alkalies, oxygen nearly pure escapes, and nitrite is formed: afterwards the nitrite undergoes decomposition, a mixture of oxygen and nitrogen passing off, and in most cases the pure oxide of the metal is left. When thrown on glowing coals, the nitrates are decomposed with scintillation: if paper be moistened with the solution of any nitrate, allowed to dry, and then burned, the DETECTION OF NITRIC ACID. 95 smouldering combustion characteristic of touch-paper will be produced. This property is, however, also exhibited by the salts of some other acids, of which the chloric is the most important. All the nitrates, when heated with sulphuric acid, evolve nitric acid; but there is no ready method of precipitating nitric acid from its solutions, since all its compounds are dissolved by water more or less freely. Various indirect methods have been proposed for ascertaining its presence.* One of the best of these consists in neutralizing the solution, if acid, with potash, and evaporating nearly to dryness: then adding a few copper clippings, and heating the mixture with a little oil of vitriol: the copper decomposes the nitric acid if present, and characteristic red fumes of peroxide of nitrogen show themselves. A quantity of these fumes, too small. to be visible, may be rendered evident by suspending in the vessel a piece of paper moistened with a mixture of starch and solution of iodide of potassium, which will become blue from liberated iodine. A still smaller quantity of the acid may be detected by mixing a small quantity of a concentrated solution of green sulphate of iron with the liquor to be tested, and allowing the oil of vitriol to flow gradually into the solution so The accurate quantitative determination of nitric acid when mixed with other acids is a matter of considerable difficulty. One method consists in the conversion of the acid into ammonia, and the subsequent determination of the amount of ammonia found (369). Another, which, when the quantity of nitric acid is very small, furnishes excellent results, is that proposed by Pugh (Q. J. Chem. Soc. xii. 35). It is based upon the determination of the amount of stannous chloride which is converted into stannic chloride when the solution is heated with nitric acid in presence of an excess of hydrochloric acid. A certain quantity of the concentrated solution containing the nitric acid to be determined is introduced into a strong tube, and a known volume of a solution of stannous chloride in a large excess of hydrochloric acid is added, the strength of the tin solution having been ascertained by the use of a standard solution of the acid chromate of potassium. Care is taken to employ an excess of the tin solution. A fragment of marble is dropped into the tube so as to produce a quantity of carbonic anhydride sufficient to displace the atmospheric air. The tube is then carefully sealed and exposed for about a quarter of an hour to a temperature of 340°. It is allowed to cool, and the contents of the tube are next transferred to a glass and diluted with three or four ounces of water; a few drops of a weak solution of iodide of potassium and starch are then added, and the amount of tin still remaining in the form of stannous salt is determined by the addition of a graduated solution of the acid chromate of potassium, until the liquid becomes blue from the liberation of iodine. The reaction upon which this process depends may be thus expressed : HNO, + 4 SnCl2 + 9 HCl = H1NCl + 4 SnCl ̧ + 3 H2O ; the nitrogen of the nitric acid being wholly converted into ammonia during the operation: the difference between the amount of the acid chromate originally required to peroxidize the quantity of tin solution employed, and that actually consumed after the experiment is over, yielding the data for fixing the quantity of nitric acid: 393'3 grs. of the acid chromate of potassium represent 63 of nitric acid (HN→3), or 54 of N ̧ ̧. 96 TESTS FOR NITRIC ACID-NITROUS OXIDE. as to form a distinct stratum below it. In this case the characteristic action consists in the formation, at the line of contact between the two liquids, of a brownish-red solution, the colour of which disappears on boiling; the coloration depends upon the circumstance that the nitric oxide which is formed by the deoxidizing action of one portion of the iron salt, becomes dissolved with the distinctive brown colour, in the solution of the unoxidized part of the ferrous salt; the deoxidation of the nitric acid which occurs may be represented in the following equation : Ferrous sulphate. Sulph. Nitric Ferric sulphate. Nitric Water. H2O 6 + 3 H2se, + 2 If a few drops of hydrochloric acid be added to a solution which contains free nitric acid, or a nitrate in solution, the liquid acquires the power of dissolving gold leaf. This effect, however, is produced by hydrochloric acid in solutions of the chlorates, bromates, and iodates; but the presence of these salts may be detected by other characters (382, 392, 397). 2 = 44. (364) NITROUS OXIDE, PROTOXIDE OF NITROGEN: NO Mol. Vol. or NO=22. Theoretic Sp. Gr. 1'5238; Observed Sp. Gr. 1527.—Preparation.—1. If a mixture of equal parts of nitric and sulphuric acid, diluted with 8 or 10 parts of water, be digested on metallic zinc, the metal displaces hydrogen, which at the moment of its liberation deoxidizes the nitric acid, and a colourless gas is slowly given off, composed of 2 atoms of nitrogen united with 1 of oxygen. 2. But to obtain the gas in a pure state it is far better to heat nitrate of ammonium (H,NNO, the salt furnished by neutralizing pure nitric acid with carbonate of ammonium) in a glass retort; the salt quickly melts, and at a temperature of between 400° and 500° apparently begins to boil, but in reality it is undergoing decomposition, by which it is entirely resolved into the gascous nitrous oxide and steam. The temperature must be carefully watched, and not be allowed to rise so high as to occasion the production of white vapours in the retort, because the decomposition is then apt to occur with explosive violence. The reaction may be explained as follows:-Ammonium is a compound of nitrogen with hydrogen; when the nitrate of ammonium is heated, the hydrogen of the ammonium combines with part of the oxygen of the nitric acid, forming water, whilst the nitrogen of the ammonium at the same time becomes oxidized at the expense of another part of the oxygen of the nitric acid. The result is |